[time-nuts] Changing ADEV, (was Phase, One edge or two?)
warrensjmail-one at yahoo.com
Fri Oct 24 20:29:41 UTC 2014
Bob Camp posted (Wed Oct 22 20:38:47 EDT 2014)
Re: [time-nuts] Phase, One edge or two?
>ADEV most certainly does change with time, even for short tau's.
Can you elaborate?
Such as when, why, what kind of change, how much change,
at how short of tau's, over how long of time,
and using what type Oscillators?
Do you know what in the freq or Phase plot is causing the ADEV to change?
There all kinds of Oscillator things that change over time
and/or need a lot of time to properly measure,
but I was under the impression that this is not the case for short tau ADEV.
Of the many OCXO type Oscillators that I've tested (HP10811 & MV89),
seldom have I seen any significant change (say greater than 10%),
in the short tau (0.01 sec to 1 sec) ADEV values, after the systematic
type errors are removed. (even when starting soon after turn on)
ADEV is used to measure random types of noise so there are of
course the statistical uncertainty variations that are a function of
the number of valid data points. I find that using a minimum of
a thousand points at each tau gives good consistent results.
What I have seen when measuring the ADEV on time nut type Osc,
is that it generally takes only a couple of minutes to get enough
samples to reliability predict what the short term tau is when
using a fast (120 sps), high resolution (0.1ps) tester.
Trouble shooting hint:
If the 0.1 sec to 10 second ADEV tau plot is not flat on a good
time nut type undisciplined OXCO, then the first places to look
for problems is in the tester, or the setup, or the procedure,
not the oscillator.
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